This hasn't been the case for over 20 years on most planes and the onboard phones were so expensive that only business or 1st class passengers would even consider using then. As usual, their convenience was valued more than everyone else's. I'm sure nowadays if a 1st class passenger got on a call over the in-flight wifi, the staff wouldn't bother them either.
With the ubiquity and generally reasonable pricing of in-flight WiFi, I'm actually a bit surprised that you don't see more people jabbering away. I've never bought WiFi myself and I suspect there are quality issues and I assume some ports may be blocked, but I haven't seen many people obviously on active calls at all.
Then why did airplanes used to have built-in phones in every seat?